Self-powered light-induced plating of metals on crystalline silicon solar cells

Solar Energy 173 (2018) 277–282

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Solar Energy journal homepage: www.elsevier.com/locate/solener

Self-powered light-induced plating of metals on crystalline silicon solar cells a

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Xiaobin Luo , Junjie Li , Xu Chen , Muhammad Sajjad , Taoling He , Xuan Li , Lijuan Wu , ⁎ Yongjun Liub, Li Sunc, Yang Rena, Xiaowei Zhoua, Zhu Liua, a b c

School of Physics Science and Astronomy, Yunnan University, Kunming City, Yunnan Province 650091, China Advanced Analysis and Measurement Center of Yunnan University, Kunming City, Yunnan Province 650091, China Department of Mechanical Engineering and Texas Center for Superconductivity (TcSUH), University of Houston, Houston, TX 77204, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Self-powered electrochemical reaction Electrolyte/Si interface Light-induced plating Silicon solar cells

Light induced deposition of metallic Zn, Fe, Co, Ni, Bi, and Cu on crystalline Si solar cell cathodes has been studied as a part of developing self-powered devices. The direct metal deposition relies on the relative position of Si Fermi level and the equilibrium potential of the metal/ion and H+/H2 in electrolyte. Direct deposition of above mentioned metals shows three types of light-induced plating (LIP) behavior. Even with a relative large potential barrier, Cu2+/Cu and Bi3+/Bi could be deposited by the induced plating of the hydrogen evolution. The current density of the Co2+/Co and Ni2+/Ni have been reduced by the light absorption of the electrolyte impact on the solar cell. This work demonstrated the Si solar cells can be used to provide self-powered for metal deposition which can be used in the heavy metal treatment for the polluted water.

1. Introduction Self-powered devices have been demonstrated in applications such as artificial synthesis (Liu et al., 2015), sensors (Lin et al., 2016), water splitting (Reece et al., 2011) and energy storage (Chen et al., 2012; X. Xu et al., 2015) using solar cell based devices. These self-powered devices mostly use thin film (Khaselev and Turner, 1998), perovskite (J. Xu et al., 2015) or dye-sensitized types of solar cells (Guo et al., 2012). These solar cells suffer from low stability and low energy efficiency issues. In comparison, crystalline Si solar cells have demonstrated better stability and higher energy conversion efficiency (20.0%). Also with its mass availability, crystalline Si can be the best candidate for energy capture self-powered devices (Green et al., 2016). So far, crystalline Si solar cells based self-powered devices have been explored for CO2 reduction (Wang et al., 2016) and light-induced plating (LIP) metallization of Si substrates (Bartsch et al., 2010; Hsiao et al., 2015; Lennon et al., 2013; Rehman and Lee, 2014; Su et al., 2012). For self-powered LIP, solar cells harvest light energy to generate electrons, and transfer them to metal ions at the interface and then directly reduce metal ions to form deposit on the Si surface (Geisler et al., 2015; Hsiao and Lennon, 2013; Huang, 2016; Huang et al., 2015; Mondon et al., 2014; Su et al., 2015; van Dorp et al., 2014; Zhou et al., 2014). During this process, Si solar cell provides direct power to collect and reduce metal ions in electrolytes, which means that the metal ion solution and the Si solar cells together form a self-powered device. In

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addition, this process can be very useful for heavy metal treatment in the polluted water. To better understand the mechanism of the metal reduction and deposition on Si solar cells via such a self-powered process, detail studies of electrolytes, electrolyte/substrate interfaces and deposition conditions of different metals are needed. In this paper, we report on a systematic study of different metal ions reduction and metal deposition on the semi-finished Si solar cells via the self-powered LIP process with no external power supply. The selfpowered LIP process has been successfully realized for Zn, Fe, Co, Ni, Bi and Cu on semi-finished crystalline silicon solar cells. Such self-powered devices can be further developed for water treatment utilizing commercial Si solar cells. 2. Experiments All experiments were carried out on commercially available semifinished silicon solar cell substrates (Voc = 0.60 V) with no silver frontside electrodes and no SiNx antireflection layers (ARC). The doping concentration in this n-doped emitter layer on the p-substrate is around 1019/cm3. For a typical experiment, the substrates were first cleaned in acetone and ethanol, followed by rinsing in deionized water for three times and then ultrasonically cleaned in 5 wt% HF for 30 s. Table 1 summarizes the electrolytes, including their concentrations and pH values used in the current self-powered LIP study. All metal ions solutions were prepared by dissolving corresponding salts in DI water

Corresponding author. E-mail address: [email protected] (Z. Liu).

https://doi.org/10.1016/j.solener.2018.07.044 Received 28 March 2018; Received in revised form 29 June 2018; Accepted 17 July 2018 0038-092X/ © 2018 Elsevier Ltd. All rights reserved.

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electrolyte (Memming, 2007; Morrison, 1980; Rajeshwar, 2007; Schmickler and Santos, 2010), two types of space charge layer configuration can be formed (Fig. 2). Here, the double layer between the Si and the electrolyte was not considered in this paper. For EF < EF, redox (EF and EF, redox are the Fermi level of the semiconductor and the equilibrium potential of the redox system, respectively), as shown in Fig. 2(a), when Si and electrolyte reach equilibrium, both the conduction band and valence band bend downwards to form an electron accumulation layer (Fig. 2b). In addition, with light on, the p/n junction of the Si solar cells, on the right side in Fig. 2(c), provides an additional potential on the n-Si/electrolyte interface to raise the Fermi level in the n-Si, resulting in metal deposition. When EF > EF, redox, as shown in Fig. 2(d), after Si/electrolyte reached to equilibrium, both the conduction band and the valence band bend upwards and form an electron depletion layer with a potential barrier qϕns (Fig. 2(e)). This potential barrier height is determined by qϕns = EF, redox − EC, which is an analogue to the metal/semiconductor interface (Sze and Ng, 2006; Tung, 1992). Furthermore, with light-on, the Fermi level of the n-Si bulk can rise up to reduce the potential barrier of qϕns, and will result in metal deposition on Si, as shown in Fig. 2(f). Hence, the open circuit voltage of Si solar cells is large enough to provide the self-powered voltage for the metal deposition both the condition of Fig. 2(a and c). Fig. 3 shows the energy levels between different metal redox couples and n-Si. Herein, the Fermi energy level of n-Si is around 0.03 eV below the conduction band (with the doping concentration of 1019/ cm3). The EF, redox of Zn2+/Zn and Fe2+/Fe redox couples are above the Fermi energy level of n-Si, which can be described by Fig. 2(a). As described by Fig. 2(c), with light turned on, Fe and Zn can directly deposit on the n-Si. In contrast, Ni2+/Ni, Co2+/Co, Cu2+/Cu and Bi3+/ Bi solutions all have EF, redox below the Fermi energy level of n-Si. Their band structures are described by Fig. 2(d), for which metal deposition challenging, especially for Cu and Bi since their qϕns values are larger than 0.6 eV which is the open circuit voltage of self-powered Si solar cells.

Table 1 Electrolyte composition and pH values used in this study. Solution

CuSO4

FeSO4

Bi(NO3)3

NiSO4

ZnSO4

CoSO4

Concentration pH

0.2 M 4.01

0.4 M 3.61

0.4 M −0.46

0.5 M 3.80

0.4 M 3.62

0.2 M 4.12

without adding buffers except for Bi(NO3)3, in which HNO3 was added to improve the solubility of Bi(NO3)3. To study the self-powered LIP process, a two-electrode configuration, in which the Si solar cell substrate was the working electrode, and the respective metal sheet (Zn, Fe, Co, Ni, Bi or Cu) was used as counter electrode to form the electrochemical cell. A fluorescent lamp (40 W) with a light intensity of 30 mW/cm2 was used and illumination time was set at 500 s for all experiments. Current generated by the selfpowered LIP process (amperometric i-t curve) was measured using an electrochemical workstation (CHI 660D). In the meanwhile, light absorption properties of the electrolytes were determined by UV/Vis spectra measurements using a Specord 200 spectrophotometer. In order to reveal the effect of solution concentration on plating rate, a series of CoSO4 solutions with different concentrations (0.05 M, 0.1 M, 0.2 M, 0.3 M, 0.4 M and 0.5 M) were prepared and measured as references. The crystalline structure of the plated metal films were characterized by X-ray diffraction (XRD) using a TTR 111 diffractometer operated at 30 KV with Cu Kα radiation (λ = 1.5065 Å). The microstructure of the samples were characterized using an FEI Quanta 200 Scanning Electron Microscope (SEM). 3. Results and discussion The schematic structure of the self-powered metal deposition cell is shown in Fig. 1. For this device, the semi-finished solar cells provide the deposition power through light conversion. The electron doped n-type Si side was in contact with solution for the metal deposition under illumination. The light-induced electrodeposition of metals are found to be strongly correlated to the interface between n-Si and electrolyte.

3.2. Self-powered light induced metals deposition on Si solar cells

3.1. The semiconductor-electrolyte interface

The current density of the self-powered light induced metal deposition is not only related to the relative position of the Fermi-level of the Si and the equilibrium potential of the electrolyte, but also related to the concentration and the light absorption of the electrolyte. To understand the metal ion concentration influence on the energy barrier, self-powered Co deposition from solutions with different Co concentration have also been studied. As shown in Fig. 4, based on Nernst equation, the equilibrium potential shifts from −0.32 V to −0.29 V (vs. SHE) as the concentration of the Co solution changes from 0.05 M to 0.5 M, corresponding to an energy barrier variation from 0.13 eV to 0.16 eV. As seen in Fig. 4(a), the deposition current rises when the light is on. When the Co ion concentration in the electrolyte increases, the corresponding light-on current density also increases. The photocurrent densities at 80 s were extracted for comparison and shown in Fig. 4(b). A linear relationship between the light-on current density and the Co concentration indicates that the current is diffusion controlled (Bard and Faulkner, 2001). Fig. 5 shows the photocurrent densities measured for different metals deposition with and without illumination. Before light-on, no metal deposition occur on the Si substrate. For all metal ion solutions, current rapidly rises when the light switches on, accompanied by metal deposition. Among them, the light-on current density of Bi3+/Bi is extremely high, while the light-on current density of Ni2+/Ni is the lowest. As discussed previously, Zn2+/Zn and Fe2+/Fe have their redox energy levels above the n-Si Fermi level (Fig. 3), with light-on, the metals of Zn and Fe will deposit as expected shown in Fig. 2(a–c). As

When the silicon p/n junction is brought into contact with an electrolyte containing a redox couple, based on the relative position of the semiconductor Fermi level and the equilibrium potential of

Fig. 1. Schematic diagram of the self-powered LIP process. 278

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Fig. 2. Schematic illustration of the band structure of the self-powered LIP process on silicon solar cell substrate, EC is the conduction band of the p/n junction, EV is the valence band of the p/n junction; (a and d) show the energy band between the p/n junction and solution before contact, (b and e) show energy band between p/n junction and solution at the interface after contact without the illumination, (c and f) show energy band between p/n junction and solution at the interface with the illumination.

with a pH value of −0.46. The corresponding potential barrier qϕns for hydrogen evolution H+/H2 is calculated to be 0.48 eV, and situates close to the Si conduction band, whereas the potential barrier qϕns of Bi3+/Bi is 0.64 eV. Hence, the abnormally high current density from the Bi3+/Bi self-powered LIP process can be partially contributed to hydrogen evolution. Hydrogen evolution also occurs during induced Cu plating but with a much lower light-on current density due to the high solution pH value and higher potential barrier. We believe that Bi and Cu deposition is actually induced by the codeposition with hydrogen, which is similar to the induced plating of alloys such as W-Co, and Ni-W (Bard and Faulkner, 2001; Brenner, 1963). Obviously, electrolyte itself can impact the self-powered metal deposition process through light absorption. As shown in Fig. 6, for transparent solutions such as Bi(NO3)3 or ZnSO4, there is little absorption in the wavelength between 350 nm and 1100 nm. Si band gap at room temperature is around 1.12 eV, corresponding to a wavelength value of 100 nm, thus for solution absorption peaks located in the short wavelength regions such as FeSO4 (around 900 nm), CuSO4 (over 600 nm), light absorption of the solution will affect Si solar cells energy conversion efficiency. In addition, the light absorption in the near-infrared region can increase solution temperature, which can have enhancing effect to self-powered LIP. During Co2+/Co and Ni2+/Ni selfpowered processes, there is a strong light absorption in the wavelength range of 430–580 nm for CoSO4 and two absorption regions (350–450 nm) and (600–800 nm) for NiSO4, which will result in significant current reduction that the output voltage of the Si solar cells in this experiment has been strongly reduced. Hence, the solutions has strong effect on the current of the Cu2+/Cu, Fe2+/Fe, Co2+/Co and Ni2+/Ni self-powered LIP. Henceforth, the extremely higher current of the Bi3+/Bi self-powered LIP process is due to the low pH value induced hydrogen evolution. However, the low current density of the Co2+/Co and Ni2+/Ni self-powered LIP light-on is due to the light absorption of the solution.

Fig. 3. The schematic diagram of energy levels for different metal redox couples and the n-Si. Here the Fermi level of Si is −0.42 eV vs. SHE (for a n-dopant concentration of 1019/cm3).

predicted by Fig. 2(d) and Fig. 3, for Ni2+/Ni and Co2+/Co, the selfpowered Si solar cells (0.60 V) can also provide enough voltage to overcome the potential barrier for Co and Ni deposition as light-on. In the meanwhile, Cu2+/Cu and Bi3+/Bi also have high light-on current density, which are 0.60 mA/cm2 and 7.18 mA/cm2, respectively. This unique light-on current density of the Bi3+/Bi and Cu2+/Cu could be contributed to several factors including hydrogen evolution and the light absorption from solution. For all metal ions self-powered LIP process, hydrogen evolution has been observed during light-on. Bi3+/Bi has specially high hydrogen yield in this self-powered LIP process and bubbles could be easily observed once the light was switched on. The Bi solution is very acidic 279

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Fig. 6. The plot of light absorption vs. wavelength in the UV–Visual spectrum for different electrolytes.

layers of Fe, Bi, Ni, Zn, and Co can be formed on the semi-finished Si solar cells substrates while the Cu deposition mainly occurs on a small fraction of the tips of the Si pyramid. These Cu deposits show well defined facets which is an indication of single crystalline nature of these islands. For Zn plating, needle-like structures have been observed, indicating the diffusion control growth, corresponding to high current density. If we ignore the surface state of the Si at the interface of the solution and the Si solar cells and double layer capacity of the solution, the selfpowered LIP metal on the Si solar cells mainly depends on the relative position of the Fermi level and the equilibrium potential of the electrolyte solution and the hydrogen evolution. As shown in Fig. 9, for Type I: qϕns < 0 and EF, redox − E (H+/H2) < 0, as shown in Fig. 2(a), the self-powered LIP metal can be initiated with light-on when there is no potential barrier. For Type II: 0 < qϕns < qϕns (H+/H2) and EF, + redox − E(H /H2) < 0, as shown in Fig. 2(d), the self-powered LIP metal also can occur as the light turned on, this can be attributed to the light generating output voltage from the semi-finished Si solar cells to help electrons overcome the potential barrier at the interface. For Type III: qϕns > qϕns (H+/H2) and EF, redox − E(H+/H2) > 0, as shown in Fig. 2(d), the voltage generated by the semi-finished Si solar cells is not high enough for self-powered LIP of metals, but the voltage is enough for initiate the hydrogen evolution. With the hydrogen evolution dominates cathodic reaction, metal co-deposition can be triggered (Paunovic et al., 2010).

Fig. 4. Self-powered LIP Co with different ion concentrations, total illumination time was 100 s. (a) is the current-time curves of self-powered LIP with different concentrations; (b) is the Co current densities at 80 s extracted from (a) and straight line fitting for the points.

4. Conclusion In summary, it is observed that when a silicon p/n junction is brought in contact with an electrolyte containing a redox couple, with light, three types of self-powered metal deposition can arise and lead to metal deposition. Type I: With EF < EF, redox (Zn2+/Zn and Fe2+/Fe), both the valance and conduction bands of Si bend downward to create an electron accumulation layer and result in easy deposition of these metals. When EF > EF, redox, there forms a potential barrier qϕns, this lead to a Type II situation where 0 < qϕns (Co2+/Co, Ni2+/Ni) < qϕns (H+/H2), self-powered LIP of metal can still exist when the output voltage from the Si solar cells is large enough to overcome the potential barrier; Then there is a Type III band structure where qϕns (Bi3+/Bi, Cu2+/Cu) > qϕns (H+/H2), under such conditions, the hydrogen evolution will become the dominant electrochemical process and hydrogen evolution can result in induced metal deposition. In above discussion, the surface state of the Si at the interface of the solution and the Si solar cells and the double layer capacity of the solution were ignored. The current of the self-powered LIP is also related to the ion concentration and the light absorption of the electrolyte. For Co2+/Co selfpowered LIP, the current is diffusion controlled. The light absorption of

Fig. 5. The current-time curves of self-powered light-induced plating for different metals.

Crystalline structure of the different metals deposited by the selfpowered LIP had been further examined by XRD. Due to the rough surface of the semi-finished Si solar cells, no obvious Si diffraction peaks were observed in the 2θ range between 10° and 67° (Fig. 7). On the other hand, for each deposit a characteristic metal diffraction peak can be observed from individual XRD diffraction pattern. Moreover, in the case of Fe2+/Fe self-powered LIP, a diffraction peak corresponding to Fe2O3 is also observed due to the fast Fe oxidization. The surface morphology of the different metal deposits formed by self-powered LIP had been studied by SEM. As shown in Fig. 8, uniform 280

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Fig. 7. The XRD patterns of silicon substrate and different metal layers deposited on the substrate.

the solution larger than the band gap of the Si, could impact the output voltage of the solar cell, hence, influence the current density of the selfpowered deposition. This systematic study of the self-powered metal could be used as the future fabrication technique for the front electrodes in the Si solar cells and the heavy metal treatment for the water pollution.

Acknowledgments We gratefully acknowledge the financial support from the Natural Science Foundation of China (grant Nos. 61664009 and 51771169) and the Youth Project of Applied Basic Research of Yunnan Science and Technology Department (grant No. 2015FD001) of Yunan Province. This work is also funded by the High-end Scientific and Technological Talents Project of Yunnan Science and Technology Department (grant No. 2013HA019) of Yunnan Province.

Fig. 9. Phase diagram of the self-powered LIP metal. Here, qϕns = EF, EC.

redox −

Fig. 8. SEM images of different metal plating layers obtained by the self-powered LIP on the silicon substrate. 281

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